This invention relates generally to radar systems, and more specifically to a radar system which is capable of synchronization with a digital elevation map (DEM) to accurately determine a location.
The proper navigation of an aircraft in all phases of its flight is based to a large extent upon the ability to determine the terrain and position over which the aircraft is passing. In this regard, instrumentation, such as radar systems, and altimeters in combination with the use of accurate electronic terrain maps, which provide the height of objects on a map, aid in the flight path of the aircraft. Electronic terrain maps are well known and are presently used to assist in the navigation of aircraft.
Pulse radar altimeters demonstrate superior altitude accuracy due to their inherent leading edge return signal tracking capability. The pulse radar altimeter transmits a pulse of radio frequency (RF) energy, and a return echo is received and tracked using a tracking system. The interval of time between signal bursts of a radar system is called the pulse repetition interval (PRI). The frequency of bursts is called the pulse repetition frequency (PRF) and is the reciprocal of PRI.
FIG. 1 shows an aircraft 2 with the Doppler effect illustrated by isodops as a result of selection by the use of Doppler filters. The area between the isodops of the Doppler configuration will be referred to as swaths. The Doppler filter, and resulting isodops are well known in this area of technology and will not be explained in any further detail. Further, the aircraft 2 in the specification will be assumed to have a vertical velocity of zero. As is known, if a vertical velocity exists, the median 8 of the Doppler effect will shift depending on the vertical velocity. If the aircraft 2 has a vertical velocity in a downward direction, the median of the Doppler would shift to the right of the figure. If the aircraft 2 has a vertical velocity in an upward direction, the Doppler would shift to the left of the figure. Again, it will be assumed in the entirety of the specification that the vertical velocity is zero for the ease of description. However, it is known that a vertical velocity almost always exists.
Radar illuminates a ground patch bounded by the antenna beam 10 from an aircraft 2. FIG. 1a shows a top view of the beam 10 along with the Doppler effect and FIG. 1b shows the transmission of the beam 10 from a side view. To scan a particular area, range gates are used to further partition the swath created by the Doppler filter. To scan a certain Doppler swath, many radar range gates operate in parallel. With the range to each partitioned area determined, a record is generated representing the contour of the terrain below the flight path. The electronic maps are used with the contour recording to determine the aircraft""s position on the electronic map. This system is extremely complex with all the components involved as well as the number of multiple range gates that are required to cover a terrain area. As a result, the computations required for this system are very extensive.
In addition to the complexity, the precision and accuracy of the distance to a particular ground area or object has never been attained using an airborne radar processor.
In one aspect, a method for determining a position of a doppler radar target in aircraft body coordinates is provided. The method comprises calculating values for doppler circle equations, in doppler coordinates, based upon a range to the target, a vehicle velocity, and a center frequency and bandwidth of a doppler swath filter and calculating an interferometric circle in body coordinates based upon a range to the target, and an interferometric angle. The method continues as the doppler circle equations are transformed into body coordinates utilizing received pitch, roll and yaw information and an intersection of the interferometric circle equations with the transformed doppler circle equations is calculated, the intersection being the position of the target in body coordinates.
In another aspect, a processor for determining a position of a doppler radar target in aircraft body coordinates is provided. The processor is configured to calculate values for doppler circle equations, in doppler coordinates, based upon a range to the target, a vehicle velocity, and a center frequency and bandwidth of a doppler swath filter, calculate an interferometric circle in body coordinates based upon a range to the target, and an interferometric angle, transform the doppler circle equations into body coordinates utilizing received pitch, roll and yaw information, and calculate an intersection of the interferometric circle equations with the transformed doppler circle equations, the intersection being the position of the target in aircraft body coordinates.
In yet another aspect, a radar signal processing unit is provided. The unit comprises a radar gate correlation circuit configured to sample radar return data from left, right, and ambiguous radar channels at a sampling rate, a correlation bass pass filter configured to stretch the sampled radar return data to a continuous wave (CW) signal, and a mixer configured to down sample an in-phase component and a quadrature component of the CW signal to a doppler frequency. The unit further comprises a band pass filter centered on the doppler frequency, and a phase processor configured to receive processed radar return data from the band pass filter. The phase processor is configured to determine a phase difference between radar return data from an ambiguous channel and a left channel, a phase difference between radar return data from an right channel and the ambiguous channel, and a phase difference between radar return data from the right channel and the left channel. The unit also comprises a processing unit configured to receive the three phase differences, adjust a phase bias for the three phase differences, resolve phase ambiguities between the three phase differences to provide a signal, and filtering the signal to provide a physical angle to a target, and a processor configured to determine a position of the target in aircraft body coordinates.
In still another aspect, a body coordinate processor is provided which comprises a doppler circle equation processor, an interferometric circle equation processor, a doppler to body coordinate transformation processor, and an intersection processor. The doppler circle equation processor is configured to calculate values for doppler circle equations, in doppler coordinates, based upon a range to the target, a vehicle velocity, and a center frequency and bandwidth of a doppler swath filter. The interferometric circle equation processor is configured to calculate an interferometric circle in body coordinates based upon a range to the target, and an interferometric angle. The doppler to body coordinate transformation processor is configured to transform the doppler circle equations into body coordinates utilizing received pitch, roll and yaw information. The intersection processor is configured to calculate an intersection of the interferometric circle equations with the transformed doppler circle equations, the intersection being the position of the target in aircraft body coordinates.
Also, a method for determining a position of a doppler radar target in body coordinates using a body coordinate processor is provided. The body coordinate processor includes a doppler circle equation processor, an interferometric circle equation processor, a doppler to body coordinate transformation processor, and an intersection processor. The method comprises calculating a radius of a doppler circle, Rd, according to Rd=target rangexc3x97sin(xcex2), and calculating a distance of the doppler circle, Xd, from the aircraft according to Xd=target rangexc3x97cos(xcex2), where xcex2=cosxe2x88x921((Fcxc3x97L)/(2xc3x97V)), and Fc is a swath filter center frequency, V is velocity, L is wavelength, and xcex2 is an angle with respect to a line of flight with the doppler circle equation processor. The method further comprises calculating an interferometric circle radius, Ri, according to Ri=target rangexc3x97cos(a), and calculating a location of the interferometric circle on a Ym axis according to Ym=target rangexc3x97sin(a) with the interferometric circle equation processor, the processor configured to receive a target range and an interferometric angle, a. The method also comprises determining a velocity vector in body coordinates, from navigation data, N, (in pitch, roll, and yaw) according to                     "LeftBracketingBar"                                                            V                X                N                                                                                        V                Y                N                                                                                        V                Z                N                                                    "RightBracketingBar"            ⁢              "LeftBracketingBar"                  TRANSPOSE          ⁢                      xe2x80x83                    ⁢          MATRIX                "RightBracketingBar"              =          "LeftBracketingBar"                                                  V              X              BODY                                                                          V              Y              BODY                                                                          V              Z              BODY                                          "RightBracketingBar"        ,
where the transpose matrix is given by       "LeftBracketingBar"                                                      cos              ⁡                              (                ψ                )                                      ⁢                          cos              ⁡                              (                θ                )                                                                                        -                              sin                ⁡                                  (                  ψ                  )                                                      ⁢                          cos              ⁡                              (                θ                )                                                                          sin            ⁡                          (              θ              )                                                                                                      cos                ⁡                                  (                  ψ                  )                                            ⁢                              sin                ⁡                                  (                  θ                  )                                            ⁢                              sin                ⁡                                  (                  φ                  )                                                      -                                          sin                ⁡                                  (                  ψ                  )                                            ⁢                              cos                ⁡                                  (                  φ                  )                                                                                                                        -                                  sin                  ⁡                                      (                    ψ                    )                                                              ⁢                              sin                ⁡                                  (                  θ                  )                                            ⁢                              sin                ⁡                                  (                  φ                  )                                                      -                                          cos                ⁡                                  (                  ψ                  )                                            ⁢                              cos                ⁡                                  (                  φ                  )                                                                                                        -                              cos                ⁡                                  (                  θ                  )                                                      ⁢                          sin              ⁡                              (                φ                )                                                                                                                    cos                ⁡                                  (                  ψ                  )                                            ⁢                              sin                ⁡                                  (                  θ                  )                                            ⁢                              sin                ⁡                                  (                  φ                  )                                                      +                                          sin                ⁡                                  (                  ψ                  )                                            ⁢                              sin                ⁡                                  (                  φ                  )                                                                                                                        cos                ⁡                                  (                  ψ                  )                                            ⁢                              sin                ⁡                                  (                  φ                  )                                                      -                                          sin                ⁡                                  (                  ψ                  )                                            ⁢                              sin                ⁡                                  (                  θ                  )                                            ⁢                              sin                ⁡                                  (                  φ                  )                                                                                                        -                              cos                ⁡                                  (                  θ                  )                                                      ⁢                          cos              ⁡                              (                φ                )                                                          "RightBracketingBar"    ,
and "psgr" is azimuth, xcex8 is pitch and xcfx86 is roll, and determining velocity unit vectors, which are direction cosines, in body coordinates according to ax=Vx/(Vx2+Vy2+Vz2)xc2xd, ay=Vy/(Vx2+Vy2+Vz2)xc2xd, and az=Vz/(Vx2+Vy2+Vz2)xc2xd, with the doppler to body coordinate transformation processor. The method concludes by calculating body coordinates according to X1=Dxc3x97ax, Y1=Dxc3x97ay, Z1=Dxc3x97az, where a velocity vector D, is calculated as Rxc3x97cos(xcex2), and xcex2=cosxe2x88x921(Fcxc3x97L/2xc3x97V), where xcex2 is the doppler angle, Fc is the swath filter center frequency, R is the range to the target, V is (Vx2+Vy2+Vz2)xc2xd, and L is the wavelength of the radar, and calculating target coordinates according to y=Rxc3x97sin(A), where A is a measured phase angle in body coordinates, z=(xe2x88x92bxc2x1(b2xe2x88x924ac)xc2xd)/(2xc3x97a), where a=1+(Z1/K1)2, b=(xe2x88x924Z1xc3x97KT/(2X1)2, and c=(KT/2X1)2xe2x88x92KA, KA is calculated as (Rxc3x97cos(A))2, KB is calculated as (Rxc3x97sin(B))2, KY=(yxe2x88x92Y1)2, KT is calculated as KT=KA+KYxe2x88x92KB+X12+Z12, and x=(KAxe2x88x92z2)xc2xd, with the intersection processor.